1. Field of the Invention
The present invention relates to an apparatus, such as a high-output photomultiplier tube or image tube, having a microchannel plate. More particularly, the present invention relates to such a high-output photomultiplier tube or image tube having a plurality of sequentially arranged, or cascaded, electron multiplier microchannel plates. Still more particularly, the present invention relates to such a high output photomultiplier tube or image tube having cascaded microchannel plates which are interbonded together to resist dislocation caused, for example, by vibration and jarring of the tube. The present invention also provides a method of making such a tube.
2. Related Technology
Microchannel plates have been used in various devices to intensify low-level images. For example, in night vision devices, a photoelectrically responsive photocathode element is used to receive photons from a low-level image. In other words, the low-level image may be far too dim to view with unaided natural vision, or may be an image of a scene illuminated by invisible infrared light. Such light is rich in the night-time sky. The photocathode produces a pattern of electrons (hereinafter referred to as, "photo-electrons") which corresponds with the pattern of photons from the low-level image. This pattern of photo-electrons is introduced into a microchannel plate, which by secondary emission of electrons in a plurality of small (or micro) channels, produces a shower of electrons in a pattern corresponding to the low-level image. That is, the microchannel plate releases and emits from its microchannels a proportional number of secondary emission electrons. These secondary emission electrons form an electron shower. The shower of electrons, at an intensity much above that produced by the photocathode, is directed onto a phosphorescent screen. The phosphors of the screen produce a visible image in yellow-green light, for example, which replicates the low-level image. Understandably, because of the microchannel plate, the representative image is pixalized, or is a mosaic of the low-level image.
More particularly, the microchannel plate itself conventionally is formed from a bundle of very small cylindrical tubes which have been fused together into a parallel orientation. The bundle is then sliced to form the microchannel plate. These small cylindrical tubes of the bundle thus have their length arranged generally along the thickness of the microchannel plate. That is, the thickness of the bundle slice or plate is not very great in comparison to its size or lateral extent, however the microchannels individually are very small so that their length along the thickness of the microchannel plate is still many times their diameter. Thus, a microchannel plate has the appearance of a thin plate with parallel opposite surfaces. The plate may contain over a million microscopic tubes or channels communicating between the faces of the microchannel plate. Each tube forms a passageway or channel opening at its opposite ends on the opposite faces of the plate. Also, each tube is slightly angulated with respect to a perpendicular from the parallel opposite faces of the plate so that electrons approaching the plate perpendicularly can not simply pass through the many channels without interacting with the interior surfaces of the channels.
Internally the many channels of a microchannel plate are each defined by or are coated with a material having a high propensity to emit secondary electrons when an electron falls on the surface of the material. In addition, the opposite faces of the microchannel plate are provided with a conductive metallic electrode coating so that a high electrostatic potential can be applied across the plate. An electrostatic potential is also applied between the photocathode and the microchannel plate to move the photoelectrons emitted by the photocathode to the microchannel plate. Consequently, electrons produced by the photocathode in response to photons from an image travel to the microchannel plate in the electron pattern corresponding to the low-level light image. These electrons enter the channels of the microchannel plate and strike the angulated walls which are coated with the secondary electron emissive material. Thus, the secondary emission electrons in numbers proportional to the number of photoelectrons, exit the channels of the microchannel plate to impinge on a phosphorescent screen. An electrostatic field between the microchannel plate and the phosphor screen drives the electrons to the screen.
Understandably, the visual image produced by an image intensifier tube is an intensified mosaic image of the low-level scene. Also, because the microchannel plate is supplying a considerably number of electrons which become a part of the electron shower on the phosphorescent screen, the plate is subject to an electrical current between the metallic electrodes on its opposite faces. This electrical current between the opposite faces of a microchannel plate is known as a "strip current" and is an indication of the level of electron multiplication provided by the microchannel plate. This strip current is also an indication of the level of electrical resistance heating experienced by a microchannel plate.
Alternatively, rather than directing the electron shower from a microchannel plate to a phosphorescent screen to produce a visible image, this shower of electrons may be directed upon an anode in order to produce an electrical signal indicative of the light or other radiation flux incident on the photocathode. As will be further explained, a device making such use of a microchannel plate is generally referred to as a photomultiplier, although internally of the device, electrons are cascaded or multiplied rather than photons. The anode electrode may take the form of an array or grid of individual anodes each receiving a respective portion of the electron shower. In this case the current flow or voltage level produced at each of the individual anodes provides an electrical analog to the light or radiation flux falling on the photocathode. This electrical analog signal may be employed to produce a mosaic image by electrical manipulation for display on a cathode ray screen, for example.
Still alternatively, such a microchannel plate can be used as a "gain block" in a device having a free-space flow of electrons. That is, the microchannel plate provides a spatial output pattern of electrons which replicates an input pattern, and at a considerably higher electron density than the input pattern. Such a device is useful, for example, to detect high energy particle interactions which produce electrons. Alternatively, such a device is useful as a particle counter. In such a use, rather than having a photocathode, the device is provided with an input element which sheds an electron when a particle of interest collides with the input element. The shed electron then stimulates the emission of secondary electrons as described above, and an output current signal proportional to the number of particle interactions is produced.
Conventional image tubes and photomultiplier tubes are also known which make use of cascaded microchannel plates. That is, multiple microchannel plates are arranged in series so that the initial electrons from a photocathode, for example, fall into the first microchannel plate. From this first microchannel plate, the secondary electrons from the first plate fall into a second microchannel plate. This second microchannel plate adds its own secondary emission electrons, and provides an increasingly intense shower of electrons. This shower of electrons may flow to a third or subsequent microchannel plate for further multiplication. In this way a very high electron gain or amplification may be effected, with each initial electron falling into the first plate resulting in several hundred to several hundred thousand electrons flowing from the last microchannel plate of the cascade. This electron shower may flow to a phosphor screen for producing a visible image, or to an anode, for example. At the anode, the electron shower becomes a current in a conductor which may be processed to count initial electrons, or to generate an image electronically, for example.
With the conventional photomultiplier tubes using cascaded microchannel plates, the electrostatic potential is connected across the top electrode of the top microchannel plate and the bottom electrode of the last or bottom microchannel plate in the cascade, with reference to the direction of electron flow. Although the microchannel plates of such a conventional photomultiplier tube are in facial contact with one another and are electrically connected in series, they are not otherwise connected. Each of the microchannel plates in the cascade experiences the same strip current flow.
A conventional microchannel plate is known in accord with U.S. Pat. No. 4,737,013, issued 12 Apr. 1988, to Richard E. Wilcox. This particular microchannel plate has an improved ratio of total end open area of the microchannels to the area of the plate. As a result, the photoelectrons are not as likely to miss one of the microchannels and impact on the surface of the microchannel plate to be bounced into another one of the microchannels. Such bounced photoelectrons, which then produce a number of secondary electrons from a part of the microchannel plate not aligned with the proper location of the photoelectron, provide noise or visual distortion in the image produced by the image intensifier. The image intensifier taught by the Wilcox patent solves this problem of the conventional technology.
Other specific examples of the uses of microchannel plates are found in the image intensifier tubes of the night vision devices commonly used by police departments and by the military for night time surveillance, and for weapon aiming. However, as mentioned above, microchannel plates may also be used to produce an electric signal indicative of the light flux or intensity falling on a photocathode, and even upon particular parts of the photocathode. In other words, if a single anode is disposed at the location ordinarily occupied by the phosphorescent screen, this anode will provide a current indicative of the photons received from a low-level scene. If the single anode is replaced with a grid or array of anodes, the various anode portions of the grid or array will provide individual electrical signals which are an electrical analog of the image mosaic. Consequently, these electrical signals can be used to drive a video display, for example, or be fed to a computer for processing of the information present in the electrical analog of the image. In this way, a microchannel plate may be used in a photomultiplier tube to produce an image for indirect viewing through the further use of a video display or computer to process the electrical signals produced by the photomultiplier tube.
In view of the above, it is also easily understood that an electron multiplier tube can be considered as an image intensifier, and could be used as a detector for electronically detecting the occurrence of events which produce photons, such as collisions in a test chamber of a particle accelerator. When such an image intensifier is provided with an array of anodes, the occurrence of a signal at one of the anodes indicates the occurrence of an event, and the location and intensity of the signal can provide information about the event, including the location of the event in a field of view of the photomultiplier tube. An array of such detectors may be used to provide multiple indications of such events, and to provide comprehensive positional information about the events, including computer-generated video and graphical representations about the particle collision events occurring in a large test chamber, for example.
However, conventional technology using cascaded microchannel plates all suffer from the deficiency that some of the individual microchannel plates in a cascade of microchannel plates are generally not fully secured in the tube housing. While one or more of the cascaded microchannel plates may be secured to the housing of the tube, others of the cascaded microchannel plates are secured in position simply by their facial frictional (and electrical) contact and interface with adjacent microchannel plates in the cascade. While this construction of the conventional devices has been satisfactory for most of the image intensifier tubes and photomultiplier tubes of the conventional technology, greater uses for these devices have revealed use environments in which the "loose" microchannel plates may be vibrated or jarred out of position. That is, if an image intensifier tube, for example, is subjected to an impact or jarring which shifts one or more of the cascade of microchannel plates relative to the other microchannel plates of the cascade, then the optimal operating relationship of these cascaded microchannel plates may be disturbed. The result may be a degradation of the image quality provided by the image tube. The same applies with respect to photomultiplier tubes using cascaded microchannel plates.